1. Field of the Invention
The present invention relates to an electroluminescent display device in which residual images stored as electric charges in pixels are eliminated or made invisible on the display panel.
2. Description of Related Art
An electroluminescent (EL) display panel is generally composed of a pair of electrodes, a pair of insulation layers and a luminescent layer interposed therebetween. The display panel is driven by a driver including electrode driving circuits and a power source circuit.
It is known that polarized electric charges still remain in pixels after the panel is turned off. The relation between imposed voltages and electric charges in a pixel (Q-V characteristic) is illustrated in FIG. 15. The voltage (V) imposed on a pixel is shown on the abscissa and the electric charge (Q) stored in the pixel on the ordinate. The Q-V relation in the pixel takes a position 1 when no voltage is applied and no electric charge is stored. To drive the EL panel, a positive pulse voltage (Vth+Vm) is imposed on the pixel in a positive field and a negative pulse voltage -(Vth+Vm) in a negative field. The voltage Vth is a threshold voltage with which the pixel starts emitting light, and the voltage Vm is a modulation voltage representing an image datum. When the positive pulse voltage is imposed, the Q-V relation changes along a path 1-2-3-4. This means that when the voltage becomes zero at the position 4, there still remains a certain electric charge. When the negative pulse voltage is imposed at that point 4, the Q-V relation changes along a path 4-5-6-7-8. When the positive and negative voltages are imposed alternatively after that point, the Q-V relation changes along a periphery of a parallelogram 9-3-5-7. The EL pixel emits light on lines 9-2-3 and 5-6-7.
To turn off the EL panel (to terminate light emission from the pixel), the voltage Vth is imposed in the positive field and the voltage -Vth in the negative field. If the voltage -Vth is imposed when a pixel is at the position 4, it returns to the position 1 through the positions 5 and 6. If the voltage Vth is imposed when a pixel is at the position 8, it returns to the position 1 through the positions 9 and 2. Therefore, no polarized electric charge is left in the pixel in both cases.
However, in actual use of the EL panel, some electric charges are left in the pixels after the panel is turned off. Under this situation, when the EL panel is turned on again and a voltage to make it dark is applied, the EL panel still emits light due to the electric charges remaining in the pixels. For example, if the EL pixel is at the position 4 when the power is turned off, and the voltage -Vth which makes the pixel dark is imposed when the power is turned on again, then its position changes along a path 4-5-6-1. The EL pixel emits light in a course of its position change from the point 5 to the point 6. Similarly, if the EL pixel is at the position 8 when the power is turned off, and the voltage Vth which makes the pixel dark is imposed when the power is turned on again, then its position changes along a path 8-9-2-1. The El pixel emits light in a course of its position change from the point 9 to the point 2. Therefore, an undesirable picture image is displayed on the panel due to the remaining electric charge when the power is supplied again after the power is once turned off. Actually, the image due to the remaining charge is mixed with an image to be first displayed, and the mixed image is displayed. Though this image is displayed only for a short time right after the power is turned on, it is still undesirable and uncomfortable for a viewer.